geotiff 0.0.2

The purpose of this library is to simply read GeoTIFFs, nothing else. It should work for other TIFFs as well, I guess, but TIFFs come in many flavors, and it's not intended to cover them all. In its current state, it works for very basic GeoTIFFs, which sufficed to extract elevation data for use in the routing library. In case you want to extend the library or have suggestions for improvement, feel free to contact me, open an issue ticket or send a pull request. You might also consider the GDAL bindings for Rust. Depending on your usecase, it might be easier to use.
Documentation 
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use std::collections::{HashMap, HashSet};
use enum_primitive::FromPrimitive;
use lowlevel::*;

/// The basic TIFF struct. This includes the header (specifying byte order and IFD offsets) as
/// well as all the image file directories (IFDs) plus image data.
///
/// The image data has a size of width * length * bytes_per_sample.
#[derive(Debug)]
pub struct TIFF {
    pub ifds: Vec<IFD>,
    // This is width * length * bytes_per_sample.
    pub image_data: Vec<Vec<Vec<usize>>>,
}

/// The header of a TIFF file. This comes first in any TIFF file and contains the byte order
/// as well as the offset to the IFD table.
#[derive(Debug)]
pub struct TIFFHeader {
    pub byte_order: TIFFByteOrder,
    pub ifd_offset: LONG,
}

/// An image file directory (IFD) within this TIFF. It contains the number of individual IFD entries
/// as well as a Vec with all the entries.
#[derive(Debug)]
pub struct IFD {
    pub count:   u16,
    pub entries: Vec<IFDEntry>,
}

/// A single entry within an image file directory (IDF). It consists of a tag, a type, and several
/// tag values.
#[derive(Debug)]
pub struct IFDEntry {
    pub tag:          TIFFTag,
    pub tpe:          TagType,
    pub count:        LONG,
    pub value_offset: LONG,
    pub value:        Vec<TagValue>,
}

/// Implementations for the IFD struct.
impl IFD {
    pub fn get_image_length() -> usize {
        3
    }

    pub fn get_image_width() -> usize {
        3
    }

    pub fn get_bytes_per_sample() -> usize {
        3
    }
}

/// Decodes an u16 value into a TIFFTag.
pub fn decode_tag(value: u16) -> Option<TIFFTag> {
    TIFFTag::from_u16(value)
}

/// Decodes an u16 value into a TagType.
pub fn decode_tag_type(tpe: u16) -> Option<TagType> {
    TagType::from_u16(tpe)
}

/// Validation functions to make sure all the required tags are existing for a certain GeoTiff
/// image type (e.g., grayscale or RGB image).
pub fn validate_required_tags_for(typ: &ImageType) -> Option<HashSet<TIFFTag>> {
    let required_grayscale_tags: HashSet<TIFFTag> = [
        TIFFTag::ImageWidthTag,
        TIFFTag::ImageLengthTag,
        TIFFTag::BitsPerSampleTag,
        TIFFTag::CompressionTag,
        TIFFTag::PhotometricInterpretationTag,
        TIFFTag::StripOffsetsTag,
        TIFFTag::RowsPerStripTag,
        TIFFTag::StripByteCountsTag,
        TIFFTag::XResolutionTag,
        TIFFTag::YResolutionTag,
        TIFFTag::ResolutionUnitTag].iter().cloned().collect();

    let required_rgb_image_tags: HashSet<TIFFTag> = [
        TIFFTag::ImageWidthTag,
        TIFFTag::ImageLengthTag,
        TIFFTag::BitsPerSampleTag,
        TIFFTag::CompressionTag,
        TIFFTag::PhotometricInterpretationTag,
        TIFFTag::StripOffsetsTag,
        TIFFTag::SamplesPerPixelTag,
        TIFFTag::RowsPerStripTag,
        TIFFTag::StripByteCountsTag,
        TIFFTag::XResolutionTag,
        TIFFTag::YResolutionTag,
        TIFFTag::ResolutionUnitTag,
    ].iter().cloned().collect();

    match *typ {
        ImageType::Bilevel => None,
        ImageType::Grayscale => None,
        ImageType::PaletteColour => None,
        ImageType::RGB => Some(required_rgb_image_tags.difference(&required_grayscale_tags).cloned().collect()),
        ImageType::YCbCr => None,
    }
}